1. Field of Invention
The present invention relates to a satellite signal reception device that receives radio signals transmitted from a positioning information satellite such as a GPS satellite to acquire the current date and time, to a timepiece that has this satellite signal reception device, and to satellite signal reception method for the satellite signal reception device.
2. Description of Related Art
The Global Positioning System (GPS), which is a system for determining the position of a GPS receiver, uses GPS satellites that circle the Earth on known orbits, and each GPS satellite has an atomic clock on board. Each GPS satellite therefore keeps the time (referred to below as the GPS time or satellite time information) with extremely high precision.
A radio-controlled timepiece that adjusts the time using time information (GPS time) from GPS satellites is taught, for example, in Japanese Unexamined Patent Appl. Pub. JP-A-H10-10251.
The signals (navigation message) from the GPS satellites are transmitted in frames and subframes synchronized to a Coarse Acquisition code (C/A code) that is reset at the beginning of the week of the GPS time. If the navigation message is interpreted using this C/A code, the time passed since the beginning of the GPS time week can be known from the signal from a single satellite, and the time can be set to a precision of approximately 0.1 second.
More specifically, the GPS satellites orbit at an altitude of approximately 20,000-27,000 km. It therefore takes approximately 66.6-90 ms for signals to travel from the GPS satellite to the GPS receiver. By correcting for this transmission time, the time can be synchronized to the Coordinated Universal Time (UTC) with error on the millisecond order, and a timepiece with sufficient practical precision can be achieved.
Hardware and products designed to receive and process satellite signals for both adjusting the time and positioning require a CPU and correlator operating at a high clock rate in order to complete the positioning calculations in a prescribed time. This hardware configuration is more than what is needed to simply adjust the time, however.
If the CPU and correlator run at a high clock rate when adjusting the time, an operation that is used more frequently than the positioning function, the peak current also rises. This can lead to an excessive voltage drop causing the system to shut down. If the voltage drops significantly due to an increase in the peak current, the charge/discharge cycle repeats more frequently, the battery therefore deteriorates more quickly, and the battery life is shortened. One possible solution is to increase the size of the battery, but this also increases the size of the satellite signal reception device and makes incorporating a satellite signal reception device in a wristwatch or other mobile device difficult.